I suspect it’s for the same reason that the two most popular pen colors are black and blue. On light backgrounds, blue shows up well and doesn’t impede readabil

I suspect it’s for the same reason that the two most popular pen colors are black and blue. On light backgrounds, blue shows up well and doesn’t impede readability. So, if all your text is black by default and you want another default color that is also readable, blue’s an obvious choice. Add underlines for users working in grayscale environments.